Selective Absorber Films Research Articles

Self‐Doped TiAl Nanoparticle/AlN Metal‐Cermet Solar Selective Absorbing Films

AbstractMetal‐cermet solar selective absorber films (SSAFs) with nanoparticles embedded in dielectric matrix have been widely used in the field of solar thermal conversion. A new SSAF of TiAlN is fabricated via magnetron sputtering method. By co‐sputtering Ti and Al targets, TiAl nanoparticles are embedded in AlN matrix to form self‐doped absorber layer. And the outermost rough AlN antireflective layer further enhances the optical properties due to the optical trapping effect, as well as the heating crack extension resistance of the surface. As a result, the as‐grown SSAF displays high absorptance reaching up to 0.92 and low thermal emittance of 0.05. The selective absorption mechanism endowed by synergetic effect of intrinsic absorption and multilayers’ interference is discussed. The SSAFs optical property is stable in open‐air up to 650 °C, indicating their enormous potential for high‐temperature solar selective applications.

Spectral Selective Solar Light Enhanced Photocatalysis: TiO2/TiAlN Bilayer Films

We demonstrate that spectral selective photocatalytic multilayer films can be tailored such that they can harness the full solar spectrum for enhanced photocatalytic gas-phase oxidation of acetaldehyde. Thin films of anatase TiO2 were deposited on a thin solar absorber TiAlN film to fabricate bilayer TiO2/TiAlN films by dc magnetron sputtering on aluminium substrates. The structural and optical properties of the films were characterized by X-ray diffraction and Raman spectroscopy. The reaction rate and quantum yield for acetaldehyde removal was measured and an almost tenfold enhancement of the quantum yield was observed for the TiO2/TiAlN films compared with the single TiO2 film, on par with enhancements achieved with new heterojunction photocatalysts. The results were interpreted by a temperature-induced change of the reaction kinetics. Absorption of simulated solar light illumination resulted in a temperature increase of the TIAlN film that was estimated to be at most 126 K. We show that a concomitant temperature increase of the top layer TiO2 by 100 K shifts the water gas-surface equilibrium from multilayer to submonolayer coverage. We propose that this is the main reason for the observed enhancement of the photocatalytic activity, whereby gas phase molecules may come in direct contact with free surface sites instead of having to diffuse through a thin water film. The implications of the results for judicious control of temperature and relative humidity for efficient gas-phase photocatalysis and exploitation of selective solar absorbing films are discussed.

Influence of parameters on Al/Ti-DLC/DLC selective absorber film

ABSTRACTThis work explores the effects of target current and argon gas flow on structural and optical properties of Al/Ti-DLC/DLC selective absorber films used in solar-thermal conversion processes. The transmittance of DLC (diamond-like carbon) anti-reflection layer is affected by the sp2-C/sp3-C ratio in it. An increase in C-target current and argon gas flow is seen to induce an initial increase followed by a subsequent decrease in film transmittance. The absorptivity of Ti-DLC (titanium-containing) layer is affected by the sp2-C/sp3-C ratio. An increase in Ti-target current induces a gradual decrease in the content of sp3-C bonds in absorbing layer. A further increase in Ti-target current results in decreased absorption. It is concluded that in the Ti-DLC layer, (i) TiC grains can change absorbing layer temperature and C–C bond structure, thereby gradually increasing the ratio of sp2-C/sp3-C and (ii) TiC grains also absorb transmitted light or scatter reflected light, inducing enhanced absorption in Ti-DLC layer.

Solar selective absorbers with foamed nanostructure prepared by hydrothermal method on stainless steel

Currently, solar absorber plays a key role among all components of the concentrating solar power (CSP) system, which is considered as an important way for solar energy utilization. Due to the outstanding features of surface texture, nano-structured materials have been extensively utilized for solar energy harvesting and conversion, but most of them were synthesized by expensive or complicated techniques. Herein, for the first time, solar selective absorbers with uniquely foamed nanostructure, are grown in situ by facile hydrothermal method on stainless steel, without using any porogen or template. The selective absorber films, which comprised a large number of nanoparticle agglomerates and nanopores, have a vital role on the solar selective absorptance owing to the increased optical path as well as enhanced sunlight trapping via foamed nanostructure. They show an excellent solar thermal performance with the solar absorptance and the thermal emittance of 0.92 and 0.12, respectively. Besides, the solar absorber films also exhibit considerable solar thermal performance and benign thermal stability at high temperature. Due to the relieving internal stresses from foamed nanostructure, there is no obvious cracking within the entire structure even after heat treatment. Consequently, the hydrothermal method used in the present investigation happens to be a novel, pollution-free, low-cost, and suitable for simultaneous mass production for large-size absorber films. Moreover, the pre-treatment of substrate and post-treatment of films are also very convenient and environmental, without any complex procedure. Accordingly, the resultant films with foamed nanostructure will have a good prospect as a new-family of solar selective absorbers, which may have a tremendous potential for industrial production in the future.

A one-step sol–gel route derived Ag–CuO film as a novel solar selective absorber

A novel one-step sol–gel method to fabricate spectrally selective solar absorber film has been developed. The selectively absorbing film consists of Ag nano-particles grow in a semiconductor matrix of cupric oxide (CuO). The influence of the preparation conditions, i.e. the addition of diethanolamine (DEA) and tetraethylenepentamine (TEPA) and Ag+/Cu2+ molar ratios, on film structure and optical properties were studied. Texture properties (crystallinity, grain size, surface topography) were obtained by using transmission electron microscopy (TEM), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The results showed that the Ag nano-particles derived from TEPA had higher cystallization and better dispersibility, which were more conductive to the intrinsic absorption. Moreover, the heat treatment in the air did not oxidize the metallic Ag with the increased Ag+/Cu2+ molar ratios, but caused the longitudinal agglomeration among the nanoparticles of Ag, thus led to a cone array structure on the film surface and enhanced the intrinsic absorption. The optical properties analysis of the fabricated film revealed a normal solar absorptivity of α=0.850 and low thermal emissivity of ε=0.05. After adding an anti-reflection layer, the absorptivity had been improved to 0.917 while the thermal emittance remained unaltered. Hence Ag–CuO composite could be used as a novel candidate material for solar selective absorber films.

Electrolytic Nickel Impregnation of Porous Anodic Aluminum Oxide Films Using AC Voltage as Solar Selective Absorber

Anodized aluminum oxide films on aluminum substrates 1050 (99.5% Al) was impregnated with nickel by electrodeposition technique. In these paper, we report the synthesis of Ni using the galvanostat mode, and also some experimental modification in the frequency and voltage values, in order to optimize the amount content of Ni on aluminum substrate The data values of the total reflectance were analyzed in the visible solar spectrum and near infrared as a function of voltage, frequency and time of the impregnation process. Several experiments were performed in order to correlate these parameters with the nickel content in the bottom of the pores of the film and its reflectance properties. By morphological analysis found that samples with average nickel content of 10% of the total pore volume not exhibit good properties of a selective absorber, however the samples containing 60% of the total pore volume, present in the reflectance spectrum visible solar 5% and 90% in the infrared solar spectrum so it can be considered a good selective absorber films makes these coatings are prospects for implementation in solar collectors.

不同氧气含量下镍铬系平板集热器选择性吸收薄膜的制备和性能表征

不同氧气含量下镍铬系平板集热器选择性吸收薄膜的制备和性能表征

Influence of microstructured substrate on solar selective absorbing films

In this paper, the influence of the substrate on the performance of solar selective absorbing films is considered experimentally. The microporous structures of aluminium foils are fabricated by a two-step anodisation method and TiNxOy multilayer films are deposited on untreated aluminium foils, polished aluminium foils and porous aluminium foils respectively. It is found that the solar selective absorbing performance of multilayer films is dependent on the microstructure of substrate surfaces. The films on the substrate after polishing treatment have the lowest absorptance. The surface oxide layer, which is a porous structured layer on aluminium foil, can result in the increase of the absorptance α and the emittance ϵ of the solar films. The reflectance of the film deposited on the microstructured substrate is lower in general. Moreover, the thickness of the surface oxide layer can be moderated to decrease thermal emittance of the solar selective absorbing films.

DESIGN AND ANALYSIS OF COLORED SOLAR SELECTIVE ABSORBING FILMS WITH THREE-LAYER STRUCTURE

Using glasses with 200-nm Cu films as substrates, the colored solar energy selective absorbing films with three-layer structure had been designed by optical multilayer design software. The three layers materials were Fe doped AlN and AlN . The optical constants of Fe doped AlN films with different Fe volume fractions were calculated by the effective medium theories. The color of the absorbing films includes orange yellow, purplish pink, pink, purple, and green. The solar absorptance of each colored film is between 0.89 and 0.96. The thermal emittance is lower than 0.1. The green film has the highest brightness. The purplish pink film has the highest absorptance, while the orange yellow and purple films have the lowest emittance. The color, absorptance, emittance, and brightness of the green film at different angles of light incidence had been analyzed. When the incident angle increases from 0° to 50°, the color of the green film changes from green to blue, and its absorptance and emittance decrease.

Optical Properties and Enhanced Photothermal Conversion Efficiency of SiO2/A-Dlc Selective Absorber Films for A Solar Energy Collector Fabricated by Unbalance Sputter

Solar energy could become the most attractive alternative energy source. In this study we test an attractive new candidate material for solar energy collectors. It can be found that the higher the gas pressure is, the higher the sp2/sp3 area ratio, the greater the sputtering rate and the greater the optical absorption. The photothermal conversion efficiency of a SiO2 coating on the amorphous diamond-like carbon (a-DLC) selective absorber films deposited on the Cr/mirror like Al substrate is 93.2% as the film thickness of a SiO2 coating is 105 nm. The coatings also increase the protective properties for a longer service life. This makes the SiO2 coated a-DLC film a promising new candidate material for solar selective absorber films. The SiO2/a-DLC selective absorber films also were deposited on the Al extrusion substrates.

Preparation of solar selective absorbing coatings by magnetron sputtering from a single stainless steel target

This paper presents a method to prepare solar selective absorbing coatings by radio frequency magnetron reactive sputtering using a single stainless steel target. Stainless steel/stainless steel nitride (SS/SS-N) ceramic–metal composite (cermet) thin films were produced under varied nitrogen gas flow ratios. The solar selective absorbing films have good solar absorptance of 0.91 and thermal emittance of 0.06 at 82 °C. The refractive index ( n) and extinction coefficient ( k) of the cermet composite layers prepared in nitrogen and argon atmospheres were determined by spectroscopic ellipsometry. The films were also analyzed by different oscillator models. The results showed a significant transformation from metal to cermet as the nitrogen gas flow ratio was increased to 10%. As the nitrogen gas flow ratio was increased to 17.5%, the film became a dielectric layer that could be used as an anti-reflection layer, suitable as the outermost layer of the solar selective absorbing coatings. A theoretical solar absorptance of 0.92 was achieved by selecting an appropriate combination of three solar absorbing layers. The experimental results agreed well with the theoretical calculations. This study proved the possibility of preparing solar selective absorbing coatings with high solar absorptance by using a single stainless steel target.

Nickel electrolytic colouring of anodic alumina for selective solar absorbing films

The influence both of the waveform of the pigmentation current and of the type of electrolyte on the quality of nickel-pigmented aluminium solar-absorbing coatings was investigated. Alternating current (AC) and rectified AC current with half-wave and full-wave rectification, respectively, were applied. As a pigmentation agent, Watts and citrate electrolytes on the nickel (Ni 2+) basis were used. Thickness, weight, colour shade of the deposited coatings were recorded before and composition and microstructure ones after stripping procedure. The stripping procedure of anodic alumina was carried out in the chrome/phosphoric acid mixture. It was confirmed that the negative half wave of anodising current is responsible for nickel deposition. Morphology study revealed that the positive half wave causes softening/dissolution of the structure deposit. Complexing agent of the nickel-based electrolyte for pigmentation of anodic alumina causes high-degree arrangement of the nickel deposit at presented conditions.

X-ray emission from plasma generated by nanosecond laser irradiating tantalum

A continuum spectrum of X-rays, originating from the interaction of a moderate intensity nanosecond Nd:Yag laser (1064 nm, 9 ns, 30 Hz, 900 mJ, 1011 W/cm2) with metal targets producing plasma, is investigated. The photon emission intensity is particularly high when the plasma expands in a low-pressure gas. The photon energy is measured through selective thin absorber films employed in front of the solid state detector. The temperature of the hot electrons generated from the plasma, responsible for the continuum spectrum emission, is calculated from the fit of the X-ray spectrum with a Maxwellian distribution, and it is about 1–2 keV.

Characterization and degradation testing of nickel pigmented anodic alumina solar selective absorber films produced by alternate and reverse periodic plating technique

Nickel pigmented anodic alumina solar selective absorber films have been prepared using alternate and reverse periodic plating technique. The films show favorable characteristics, high absorptance (α = 0.97), low emissivity (ϵ = 0.16), for the use as solar selective absorber materials. The effect of the alternate and reverse periodic plating parameters on the optical and films microstructural properties has been studied. Optical and structural properties of the films are investigated using X-ray diffraction, TEM, SEM, EDS, UV/VIS/near IR. These investigations reveal that the nickel particles in alumina pores are spherical with average crystals size ranged from approximately 150–300 Å. The optical characteristics of the films with the preparation conditions are compared. The stability of the films to high temperatures and humidity has been tested. The results from these tests show that the reverse periodic plated films are more sensitive to degradation process mainly nickel oxidation.

Electrolytic coloring of anodic aluminum for selective solar absorbing films

Electrolytic coloring of anodic aluminum for selective solar absorbing films

Electrolytic coloring of anodic aluminum for selective solar absorbing films: use of additives promoting color depth and rate

An optically black coating on aluminum substrate having improved solar absorption is disclosed along with an enhanced process for formation. The two-step electrolytic coloring process has been used. Porous anodic films have been formed by dc anodization of aluminum samples in 16% sulfuric acid. It is followed by black coloration via ac electrolysis in nickel sulfate solution. In this step, we have investigated the effect of chloride and magnesium as additives. Evaluation of the test samples has been carried out by spectral reflectance and nickel content measurements. Choride existance verifies two aspects: the promotion of the coloring process and higher black tones shades are attained. However, magnesium has a minor effect.

CoAl 2O 3 selective solar absorbing films: structure and effective medium theory for the optical properties

We present a detailed picture of the structure of CoAl 2O 3 composite coatings. The structural information can be used to derive an effective medium theory, which takes into account a structural constant related to the pair and three-point correlation functions of the material. Comparisons between theory and experiments on the optical properties of CoAl 2O 3 in the 0.35 <Λ<2.6 μm wavelength range shows good qualitative agreement. The present theory shows significant improvements over the Maxwell Garnett theory which was previously used for this type of composite films.

Electrodeposition of black chrome selective solar absorber coatings with improved thermal stability

Auger electron spectroscopy was used to study the oxidation behavior of electrodeposited black chrome selective solar absorber films. The volume fraction of Cr 2O 3 increased rapidly initially, and the films then exhibited an increasingly slower rate of oxidation. The Cr 2O 3 formed during electrodeposition does not passivate the film; it is only the Cr 2O 3 formed during the initial thermal exposure that forms a passivating layer. Thus the optical properties of the as-deposited films change rapidly with heating. The deposition of larger agglomerates results in better thermal stability of the optical properties.